Rubicon, Latin Rubico, or Rubicon, small stream that separated Cisalpine Gaul from Italy in the era of the Roman Republic. The movement of Julius Caesar’s forces over the Rubicon into Italy in 49 bc violated the law (the Lex Cornelia Majestatis) that forbade a general to lead an army out of the province to which he was assigned. His act thus amounted to a declaration of war against the Roman Senate and resulted in the three-year civil war that left Caesar ruler of the Roman world. “Crossing the Rubicon” became a popular phrase describing a step that definitely commits a person to a given course of action.

The modern Rubicone (formerly Fiumicino) River is officially identified with the Rubicon that Caesar crossed, but the Pisciatello River to the north and the Uso to the south have also been suggested.

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...declared himself, and, early in 49, the Senate voted to outlaw Caesar. Two tribunes supporting him (one of them Mark Antony) had to flee. By the time they reached him, Caesar had already crossed the Rubicon: he now had a cause.

On January 10–11, 49 bce, Caesar led his troops across the little river Rubicon, the boundary between his province of Cisalpine Gaul and Italy proper. He thus committed the first act of war. This was not, however, the heart of the matter. The actual question of substance was whether the misgovernment of the Greco-Roman world by the Roman nobility should be allowed to continue or whether...